Parking brake unit for work vehicle

ABSTRACT

A brake release unit is provided between a transmission control unit for controlling a transmission and a clutch device. Pressure oil supplied from a hydraulic pressure source to the clutch device is also guided to an accumulator of the brake release unit. When the engine is stopped, pressure oil from the clutch device is collected in a reservoir, and the transmission operates as a parking brake. Pressure oil accumulated in the accumulator is kept as it is. If a solenoid switching valve of the brake release unit is driven as a result of operation of a switch in the operator&#39;s cabin, pressure oil accumulated in the accumulator is supplied to the clutch device. As a result, it is possible to easily release the parking brake.

The present application is based on Japanese patent application No.2000-273738 (filed Sep. 8, 2000), the contents thereof are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a parking brake unit for a work vehiclehaving a parking brake operated when the engine is stopped.

BACKGROUND ART

Conventionally, a negative parking brake has been provided in a workvehicle such as a wheeled hydraulic excavator or a wheel loader, inaddition to a foot brake. Operation of this parking brake is released ifrelease pressure is supplied from a hydraulic source after an engine hasbeen started, and the parking brake is operated if supply of the releasepressure is stopped when the engine is stopped.

In the case where an engine can not be started, such as because ofvehicle failure etc., the vehicle shall be towed to a specifiedlocation. In this case, if the parking brake is kept operating, itbecomes difficult to tow the vehicle away. For this reason, it isnecessary to allow the parking brake to be released even if the engineis stopped. In the related art, the parking brake is released byoperating a release lever or the like at outside of the operator'scabin, which is provided close to the parking brake. However, it istroublesome to release the parking brake manually at outside of theoperator's cabin. Also, in the case where the release lever for theparking brake is provided at a place hidden from the view, operabilityis degraded.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a parking brake unitfor a work vehicle that releases a parking brake easily with anoperation inside the operator's cabin, even when the engine is stopped.

In order to achieve the above described object, a parking brake unit fora work vehicle comprises a hydraulic source; a negative parking brakeprovided a lower portion of the vehicle, that releases brake operationin response to brake release pressure; an operating member that outputsa release command for the parking brake in response to operationperformed in an operator's cabin; and a brake release means forreleasing operation of the parking brake by leading the brake releasepressure to the parking brake if the release command is output. And, thebrake release means comprises a brake release accumulator thataccumulates pressure oil from the hydraulic source, and a control valvethat allows the pressure oil accumulated in the accumulator to act onthe parking brake as the brake release pressure if the release commandis output.

It is preferable that the parking brake is constituted by a transmissionhaving a negative first clutch device and a negative second clutchdevice. The transmission sets a first gear ratio by allowing a propellershaft to rotate if the first clutch device is released by controlpressure for clutch release, and sets a second gear ratio smaller thanthe first gear ratio by allowing the propeller shaft to rotate if thesecond clutch device is released by the control pressure for clutchrelease. It is preferable that the control valve allows the pressure oilfrom the accumulator to act on the second clutch device as the brakerelease pressure if the release command is output. The control valve mayallow the pressure oil from the accumulator to act on the first clutchdevice as the brake release pressure if the release command is output.The control valve may also allow the pressure oil from the accumulatorto act on one of the first clutch device and the second clutch device asthe brake release pressure if the release command is output.

It is also possible to further comprise a transmission control means,having a gear ratio control valve that controls flow of the controlpressure for clutch release for acting on the first clutch device andthe second clutch device, and a sudden operation prevention accumulatorthat prevents sudden operation of the parking brake, and the brakerelease means may be provided between the parking brake and thetransmission control means.

It is desirable that the brake release means further comprises aprevention means for preventing the pressure oil accumulated in theaccumulator flowing out to the hydraulic power side.

It is possible to further comprise a brake valve that generates brakeoperating pressure in correspondence with operation of a brake pedal; afoot brake unit that operates in response to the brake operatingpressure; and a foot brake accumulator that accumulates specifiedpressure oil in order to generate the brake operating pressure inaccordance with operation of the brake pedal when an engine is stopped.The foot brake accumulator may also function as the brake releaseaccumulator.

In order to achieve the above described object, a parking brake unit fora work vehicle comprises a hydraulic source; a negative parking brakeprovided a lower portion of the vehicle, that releases brake operationin response to brake release pressure; an operating member that outputsa release command for the parking brake in response to operationperformed in an operator's cabin; and a brake release means forreleasing operation of the parking brake by leading the brake releasepressure to the parking brake if the release command is output. Theparking brake is constituted of a transmission having a negative firstclutch device and a negative second clutch device. The transmission setsa first gear ratio by allowing a propeller shaft to rotate if the firstclutch device is released by control pressure for clutch release, andsets a second gear ratio smaller than the first gear ratio by allowingthe propeller shaft to rotate if the second clutch device is released bythe control pressure for clutch release. It is preferable that the brakerelease means comprises a brake release accumulator that accumulatespressure oil from the hydraulic source, and a control valve that allowsthe pressure oil accumulated in the accumulator to be guided to thesecond clutch device as the brake release pressure if the releasecommand is output.

As described above, if the release command is output in response to anoperation performed in the operator's cabin, the brake release meansallows the oil accumulated in the accumulator to flow, and leads thebrake release pressure to the parking brake. Therefore, even if theengine is stopped, it is possible to release the parking brake throughthe operation in the operator's cabin.

If the brake release command is output, pressure oil acts on the firstclutch device or the second clutch device so that a locked state of thetransmission is released. In this manner, it is not necessary to providethe parking brake separately. Also, if the gear ratio of thetransmission is set to a second gear ratio that is smaller than a firstgear ratio set by the first clutch device by guiding the pressure oil tothe second clutch device when the brake release command is output, atowing operation can be performed easily because drive torque of themotor at the time of parking brake release is small.

The brake release means is provided between the parking brake and thetransmission control means. As a result, since the pressure oil does notflow from the brake release means to the transmission control means, itis possible to minimize the brake release accumulator.

Also, by using the pressure oil from the foot brake accumulator as therelease pressure for the parking brake unit, it is possible to reducethe number of components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram for traveling adapted in a workvehicle having the parking brake unit of this embodiment.

FIG. 2 is a hydraulic circuit diagram showing the structure of a parkingbrake unit of a first embodiment.

FIG. 3 is a hydraulic circuit diagram showing the structure of a parkingbrake unit of a second embodiment.

FIG. 4 is a side elevation of a wheeled hydraulic excavator to which thepresent invention applies.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

A first embodiment of the present invention will now be described withreference to FIG. 1, FIG. 2 and FIG. 4. In the following, a descriptionwill be given for the case where the present invention is applied to awheeled hydraulic excavator. FIG. 4 is a side elevation of a wheeledhydraulic excavator. The wheeled hydraulic excavator comprises anundercarriage 81, and an upper structure 83 connected to an upper partof the undercarriage 81 via a swing device 82 so as to be capable ofrevolving. A front attachment 84 for working and an operator's cabin 85are provided on the upper structure 83.

FIG. 1 is a hydraulic circuit diagram for traveling adapted in a wheeledhydraulic excavator to which the present invention is applied. As shownin FIG. 1, direction and flow amount of delivery oil from a main pump 2driven by an engine (prime mover) 1 are respectively controlled by acontrol valve 3. Pressure oil whose direction and flow amount have beencontrolled passes through a center joint 5 and is supplied to a travelmotor 7 via a counter balance valve 6. The rotation speed of the travelmotor 7 is varied by a transmission 8, and transmitted via a propellershaft 9 and axle 10 to tires 11. In this way, the wheeled hydraulicexcavator travels.

The transmission 8 is a well-known device having a planetary reductionmechanism, and clutch devices CR1 and CR2. The planetary reductionmechanism comprises a sun gear, a planetary gear and a ring gear, withthe clutch devices CR1 and CR2 being provided at the sun gear side andthe ring gear side, respectively. Each of the clutch devices CR1 and CR2is put into engaged states by force of a spring. Also, the clutchdevices CR1 and CR2 are set to released states using hydraulic pressurefrom the hydraulic source 12 acting against the spring force. Hydraulicpressure acting on the clutch devices CR1 and CR2 is controlled inaccordance with operation of the transmission control valve 30.

If specified hydraulic pressure acts on the clutch device CR1 via thepipeline 13 in response to drive of the transmission control valve 30,the clutch device CR1 is set to the released state and the clutch deviceCR2 is set to the engaged state. As a result, a specified gear ratio R1is established and first gear travel becomes possible at low-speedhigh-torque. If a specified hydraulic pressure acts on the clutch deviceCR2 via the pipeline 14 in response to drive of the transmission controlvalve 30, the clutch device CR2 is set to the released state and theclutch device CR1 is set to the engaged state. As a result, a specifiedgear ratio R2 is established and second gear travel is enabled athigh-speed low-torque. Here, the gear ratio R1 for enabling low-speedhigh-torque is larger than the gear ratio R2 for enabling high-speedlow-torque (R1>R2). The pipeline 15 is a return pipeline.

On the other hand, if hydraulic pressure does not act on either theclutch device CR1 or the clutch device CR2 both clutch devices CR1 andCR2 are set to the engaged state by the spring force. As a result, thetransmission 8 is locked and rotation of the propeller shaft 9 isprevented. With this embodiment, prevention of rotation of the propellershaft 9 achieved by locking the transmission 8 is used as a parkingbrake. That is, the transmission 8 is a spring applied hydraulic releaseparking brake or a negative parking brake that is released if therelease pressure is supplied from the hydraulic source, and operated ifsupply of the release pressure is stopped.

A pilot circuit for traveling comprises a pilot hydraulic source 16, atravel pilot valve 18, a slow-return valve 19, and a forward and reverseswitching valve 20. The pilot hydraulic source 16, similarly to the mainpump 2, is driven by the engine (prime mover) 1 to generate pressureoil. The travel pilot valve 18 generates pilot secondary pressure inresponse to depression of the accelerator pedal 17. The slow-returnvalve 19 is connected with the pilot valve 18 and delays the oilreturning to the pilot valve 18. The forward and reverse switching valve20 is connected with the slow return valve 19 and is switched by anoperating lever 21 to select forward, reverse or neutral of the vehicletravel.

Pilot pressure from the pilot hydraulic source 16 acts on the pilot portof the control valve 3 to drive the control valve 3. Vehicle travelspeed can be adjusted by varying the stroke amount of the control valve3 using the accelerator pedal 17.

A well-known foot brake is provided in the wheeled hydraulic excavator,which exerts deceleration force in response to operation of a brakepedal 22. A pilot circuit for this foot brake comprises a pilothydraulic source 23 driven by the engine (prime mover) 1, similarly tothe main pump 2, to generate pressure oil, a check valve 24, and a brakevalve 25 for generating pilot secondary pressure in response todepression of the brake pedal 22. An accumulator 26 for accumulatingspecified pressure oil is also provided between the check valve 24 andthe brake valve 25.

Specified pressure from the hydraulic source 23 driven by the engine(prime mover) 1 is accumulated in the accumulator 26. Even if thehydraulic source 23 stops supplying pressure oil because the engine isstopped etc., pressure oil is accumulated in the accumulator 26.Therefore, it is possible to cause the foot brake to be driven byoperation of the pedal 22 for only a specified time using the pressureoil accumulated in the accumulator 26.

FIG. 2 is a hydraulic circuit diagram showing the structure of thetransmission control valve 30. As shown in FIG. 2, the transmissioncontrol valve 30 has a transmission control unit 30A for controllingdrive of the transmission 8 and a brake release unit 30B for releasingthe parking brake when the engine is stopped.

Pressure oil from the hydraulic power source 12 is guided through acheck valve 31 to a solenoid switching valve 32. Pressure oil passingthrough the solenoid switching valve 32 is guided to a solenoidswitching valve 34 (gear ratio control valve) through a pipeline 33.Pressure oil passing through the solenoid switching valve 34 is guidedto the clutch device CR1 via a pipeline 13 or to the clutch device CR2via a solenoid switching valve 35 (control valve) and a pipeline 14. Anaccumulator 37 is connected to the pipeline 33 via a restrictor 36.

A pipeline connected to the solenoid switching valve 34 and goingtowards the solenoid switching valve 35 branches into two. One pipeline38 is connected directly to the solenoid switching valve 35, while theother pipeline 39 is connected to the solenoid switching valve 35 via acheck valve 40. An accumulator 41 is connected to the pipeline 39between the check valve 40 and the solenoid switching valve 35.

A transmission drive switch 51, a gear change switch 52 and a parkingbrake release switch 53 are respectively provided inside the operator'scabin 85. These switches 51-53 are respectively connected between thesolenoids of the solenoid switching valves 32, 34 and 35 and a powersource 54. The solenoid switching valves 32, 34 and 35 are respectivelyswitched in response to operation of the switches 51-53. If an enginestart keyswitch (not shown) is turned on, it becomes possible to supplyelectricity from the power source 54, while it is prevented fromsupplying electricity from the power source 54 if the keyswitch isturned off.

Next, characteristic operations of the first embodiment will bedescribed.

(1) First Gear Travel

When causing the vehicle to travel in first gear, first of all, thekeyswitch is turned on so that a starter is driven to start the engine1. The switches 51 and 52 are then turned on and the switch 53 is turnedoff. As a result, the solenoid switching valves 32 and 34 are switchedto position a and the solenoid switching valve 35 is switched toposition b. As a result of this switching, hydraulic pressure from thehydraulic source 12 regulated by the relief valve 42 passes through thecheck valve 31, solenoid switching valve 32, pipeline 33, solenoidswitching valve 34 and pipeline 13 and acts on the clutch device CR1. Atthe same time, pressure oil that has acted on the clutch device CR2passes through the pipeline 14, solenoid switching valve 35, pipeline 38and solenoid switching valve 34, and is finally collected in areservoir. As a result, the clutch device CR1 is set to the releasedstate and the clutch device CR2 is set to the engaged state and the gearratio of the transmission 8 is set to a gear ratio R1 with low-speedhigh-torque.

With the clutch device CR1 in the released state and the clutch deviceCR2 in the engaged state, the operator switches the operating lever 21to the forward side or the reverse side, and presses the acceleratorpedal 17 down. The control valve 3 is switched in response to theoperating direction of the operating lever 21 and extent of depressionof the accelerator pedal 17 and pressure oil from the hydraulic pump 2is supplied to the hydraulic motor 7 to enable first gear travel of thevehicle. If the solenoid switching valve 32 is switched to position a,pressure oil from the hydraulic source 12 is accumulated in theaccumulator 37.

(2) Second Gear Travel

When causing the vehicle to travel in second gear, the switch 52 isturned off from the above described condition. As a result, the solenoidswitching valve 34 is switched to position b and hydraulic pressure fromthe hydraulic source 12 acts on the clutch device CR2, while pressureoil that has acted on the clutch device CR1 is collected in thereservoir. As a result, the clutch device CR1 is set to the engagedstate and the clutch device CR2 is set to the released state and thegear ratio of the transmission 8 is set to a gear ratio R2 withhigh-speed low-torque. This enables the vehicle to travel in secondgear. At this time, pressure oil from the hydraulic source 12 isaccumulated in the accumulator 41 via the check valve 40.

If the brake pedal 22 is operated during traveling, hydraulic pressurecorresponding to the extent to which the brake pedal 22 is operated isgenerated or output from the brake valve 25. The foot brake is activatedby this hydraulic pressure to reduce the speed of the vehicle.

(3) Parking Brake Operation

When engaging the parking brake to operate, the vehicle is stopped andthe switch 51 is turned off. Accordingly the solenoid switching valve 32is switched to position b, and pressure oil that was acting on eitherclutch device CR1 or CR2 passes through the solenoid switching valve 34,pipeline 33, solenoid switching valve 32 and relief valve 43 to becollected in the reservoir. At this time, pressure oil accumulated inthe accumulator 37 is guided to the pipeline 33 via the restrictor 36.In this way, pressure oil that was acting on the clutch device CR1 orCR2 is gradually reduced, thus preventing sudden operation of theparking brake.

When a specified time has elapsed after turning the switch 51 off,hydraulic pressure that was acting on either the clutch device CR1 orthe clutch device CR2 is reduced to a specified value or less and theclutch devices CR1 and CR2 both become engaged. Accordingly, thetransmission 8 is locked and the parking brake is operated. It is to benoted that pressure oil acting on the clutch devices CR1 and CR2 worksas a brake release pressure. That is, if supply of brake releasepressure is stopped, the transmission 8 acts as a parking brake.

If the keyswitch is turned off to stop the engine, supply of electricityfrom the power source 54 is cut off and the solenoid switching valves32, 34 and 35 are respectively switched to position b regardless ofoperation of the switches 51-53. Therefore, similarly to as describedpreviously, pressure oil that was acting on the clutch devices CR1 andCR2 is reduced gradually. After a specified time the clutch devices CR1and CR2 become engaged, and the parking brake is automatically operated.

Even if the engine is stopped and the parking brake is in an operatedstate, specified pressure oil remains accumulated in the accumulator 41.This is caused because the upstream side and the downstream side of theaccumulator 41 are respectively blocked by the check valve 40 and thesolenoid switching valve 35. The upstream side indicates the hydraulicsource 12 side and the downstream side indicates the transmission 8side. If the brake pedal 22 is pressed down, pressure oil accumulated inthe accumulator 26 acts on a brake cylinder for foot brake etc. As aresult, it is possible to cause the foot brake to operate for aspecified time even if the engine is stopped.

(4) Parking Brake Release

For releasing the parking brake while the engine is stopped, first ofall, the keyswitch is turned on. At this time, it is not necessary todrive the starter as long as electricity is supplied from the powersource 54 to the parking brake release switch 53. Next, the switch 53 isturned on. Accordingly, the solenoid switching valve 35 is switched toposition a, and pressure oil accumulated in the accumulator 41 acts onthe clutch device CR2 via the solenoid switching valve 35. The clutchdevice CR2 is set to the released state and the parking brake isreleased. As a result, it is possible to tow the vehicle. In this case,if the vehicle is towed, the hydraulic motor for traveling 7 is causedto rotate. Therefore, a resistive force is exerted due to the pumpoperation of the hydraulic motor 7, but since the clutch device CR2 isreleased, the transmission 8 is in the second gear state with smallresistive force, accordingly, it is possible to perform towing operationeasily.

As has been described above, according to the first embodiment, theaccumulator 41 is connected to the pipeline 14 through which pilotpressure for parking brake release passes. Furthermore, the upstreamside and downstream side of the accumulator 41 are respectively blockedby the check valve 40 and the solenoid switching valve 35 to holdpressure oil in the accumulator 41. Accordingly, even if the engine isstopped pressure oil is made to act from the accumulator 41 on theclutch device CR2 by switching the solenoid switching valve 35 thoughswitch operation performed in the operator's cabin and the parking brakeis thus released. Moreover, since the transmission 8 is endowed with thefunction of a negative parking brake, it is not necessary to provide adedicated parking brake separately, thus reducing the number ofcomponents.

The parking brake unit 30B is provided between the transmission 8 andthe transmission controller 30A. Specifically, the accumulator 41 isprovided more to the downstream side than the solenoid switching valves32 and 34 and the accumulator 37, therefore, it is possible to reducethe size of the accumulator 41 for the following reasons. For example,if the accumulator 41 is provided at an upstream side (for example,between the solenoid switching valve 32 and the check valve 31),pressure oil from that accumulator at the time of parking brake releaseis also guided to the accumulator 37. Therefore a large capacity wouldbe required for the accumulator to take into account the amount ofpressure oil guided to the accumulator 37. However, with this embodimentit is possible to minimize the accumulator 41 because the accumulator 41is provided at the downstream side and pressure oil is not guided to theaccumulator 37.

The accumulator 41 is allowed to be connected with the pipeline 14 forguiding control pressure to the clutch device CR2 for second gear sothat the transmission 8 is put into the second gear state at the time ofparking brake release. If the transmission 8 is in the second gearstate, torque for driving the hydraulic motor 7 is small compared to thefirst gear state and a towing operation is performed easily.

Second Embodiment

A second embodiment of the present invention will now be described usingFIG. 3. FIG. 3 is a hydraulic circuit diagram showing the structure of atransmission control valve 30 of the second embodiment of the presentinvention. Elements identical to that shown FIG. 2 have the samereference numerals, and the following description will focus on pointsdifferent from FIG. 2.

A transmission control valve 30 comprises a transmission control unit30A and a brake release unit 30C. As shown in FIG. 3, the structure ofthe brake release unit 30C is different from that of the firstembodiment.

An accumulator 26 for the footbrake is connected to the solenoidswitching valve 35 through the pipeline 45. Accordingly, when the engineis stopped, if the parking brake release switch 53 is turned on asdescribed above, pressure oil from the accumulator 26 is guided to theclutch device CR2 and the parking brake is released.

As described above, with the second embodiment, the accumulator 26 forthe foot brake and the solenoid switching valve 35 are connected witheach other via the pipeline 45. The parking brake is then released usingthe pressure oil accumulated in the accumulator 26. It is therefore notnecessary to separately provide the accumulator 41 and check valve 40etc., and it is possible to reduce the number of components.

The present invention is directed to release the parking brake usingpressure oil from the accumulator 41 when the engine is stopped, byconnecting an accumulator 41 to a pipeline 14 for guiding brake releasepressure and then, and this is not limited to the above describedembodiments and various modifications can be made. For example, with theabove described embodiments the transmission 8 is used to achieve thefunction of a parking brake, but it is also possible to provide anindependent negative parking brake separate from the transmission 8, andto release this parking brake using pressure oil from the accumulator41.

Furthermore, the accumulator 41 has been provided at a downstream sideof the solenoid switching valves 32 and 34 and the accumulator 37, butmay also be provided at an upstream side. Likewise, the accumulator 41has been connected to the control pipeline 14 of the clutch device CR2for second gear, but it may also be connected to the control pipeline 13of the clutch device CR1 for first gear. If the accumulator 41 isconnected to the pipeline 13, the solenoid switching valve 35 and thecheck valve 40 will also be connected to the pipeline 13.

It is also possible to connect the accumulator 41 to both of thepipelines 13 and 14 and have pressure oil flow through either thepipeline 13 or the pipeline 14 selectively. In this case, the pipeline39 connected with the accumulator 41 may be selectively connected toeither the pipeline 13 or the pipeline 14 using a solenoid switchingvalve or the like. In this manner, it is possible to select the gearratio at the time of a towing operation. For example, in the case oftowing the vehicle going downhill, by having pressure oil flow into thepipeline 13, a towing operation can be performed slowly owing to theresistive force generated by the hydraulic motor 7.

The present invention may also be applied to work vehicles other than awheeled hydraulic excavator, and also to automatic vehicles.

Advantages obtained by the parking brake used for a work vehicle of thepresent invention described above will now be described.

(1) If a release command is output in response to operation of theswitch 53, being an operating member, in the operator's cabin, thesolenoid switching valve 35 (control valve) allows oil accumulated inthe accumulators 41 and 26 to flow, and guides brake release pressure tothe parking brake 8. By forming the brake release means 30B and 30C inthis manner, it is possible to easily release the parking brake throughan operation performed in the operator's cabin even if the engine isstopped.

(2) The parking brake is constituted of a transmission 8 having a firstclutch device CR1 and a second clutch device CR2. If the brake releasecommand is output, pressure oil is guided to the first clutch device CR1or the second clutch device CR2 in order to release a locked state ofthe transmission 8. In this manner, it is not necessary to provide theparking brake separately. Also, a second gear ratio set in the secondclutch device CR2 is smaller than a first gear ratio set in the firstclutch device CR1. If the brake release command is output, pressure oilis guided to the second clutch device CR2. As a result, the gear ratioof the transmission 8 is set to the second gear ratio and drive torqueof the motor at the time of parking brake release is set small so that atowing operation can be performed easily.

(3) The brake release means 30B and 30C are provided between the parkingbrake 8 and the transmission control means 30A. As a result, pressureoil is not guided from the accumulators 41, 26 of the brake releasemeans 30B, 30C towards the transmission control means 30A, therefore, itis possible to minimize the brake release accumulator.

(4) The check valve 40 (prevention means) prevents pressure oilaccumulated in the accumulator 41 flowing out to the hydraulic source 12side. In this way, pressure oil of the accumulator 41 is reliably guidedto the parking brake 8 as parking brake release pressure.

(5) Since pressure oil from the foot brake accumulator 26 is used asrelease pressure for the parking brake, it is possible to reduce thenumber of components.

What is claimed is:
 1. A parking brake unit for a work vehicle,comprising: a hydraulic source; a negative parking brake provided alower portion of the vehicle, that releases brake operation in responseto brake release pressure; an operating member that outputs a releasecommand for the parking brake in response to an operation performed inan operator's cabin; a brake release device that releases operation ofthe parking brake by leading the brake release pressure to the parkingbrake if the release command is output; and a transmission controldevice; wherein: the parking brake is constituted of a transmissionhaving a negative first clutch device and a negative second clutchdevice, the transmission sets a first gear ratio by allowing a propellershaft to rotate if the first clutch device is released by controlpressure for clutch release, and sets a second gear ratio smaller thanthe first gear ratio by allowing the propeller shaft to rotate if thesecond clutch device is released by the control pressure for clutchrelease, the brake release device comprises a brake release accumulatorthat accumulates pressure oil from the hydraulic source, and a controlvalve that allows the pressure oil accumulated in the brake releaseaccumulator to act on the second clutch device as the brake releasepressure if the release command is output, the transmission controldevice comprises a sear ratio control valve that controls flow of thecontrol pressure for clutch release for acting on the first clutchdevice and the second clutch device, and a sudden operation preventionaccumulator that prevents sudden operation of the parking brake, and thebrake release device is provided between the parking brake and thetransmission control device.
 2. A parking brake unit for a work vehicleaccording to claim 1, wherein: the brake release device furthercomprises a prevention device that prevents the pressure oil accumulatedin the brake release accumulator flowing out to the hydraulic powerside.
 3. A parking brake unit for a work vehicle according to claim 1,further comprising: a brake valve that generates brake operatingpressure in correspondence with operation of a brake pedal; a foot brakeunit that operates in response to the brake operating pressure; and afoot brake accumulator that accumulates specified pressure oil in orderto generate the brake operating pressure in accordance with operation ofthe brake pedal when an engine is stopped, wherein the foot brakeaccumulator also functions as the brake release accumulator.
 4. Aparking brake unit for a work vehicle, comprising: a hydraulic source; anegative parking brake provided a lower portion of the vehicle, thatreleases brake operation in response to brake release pressure; anoperating member that outputs a release command for the parking brake inresponse to an operation performed in an operator's cabin; a brakerelease device that releases operation of the parking brake by leadingthe brake release pressure to the parking brake if the release commandis output; and a transmission control device, wherein: the parking brakeis constituted of a transmission having a negative first clutch deviceand a negative second clutch device, the transmission sets a first gearratio by allowing a propeller shaft to rotate if the first clutch deviceis released by control pressure for clutch release, and sets a secondgear ratio smaller than the first gear ratio by allowing the propellershaft to rotate if the second clutch device is released by the controlpressure for clutch release, the brake release device comprises a brakerelease accumulator that accumulates pressure oil from the hydraulicsource, and a control valve that allows the pressure oil accumulated inthe brake release accumulator to act on the first clutch device as thebrake release pressure if the release command is output, thetransmission control device comprises a gear ratio control valve thatcontrols flow of the control pressure for clutch release for acting onthe first clutch device and the second clutch device, and a suddenoperation prevention accumulator that prevents sudden operation of theparking brake, and the brake release device is provided between theparking brake and the transmission control device.
 5. A parking brakeunit for a work vehicle according to claim 4, wherein: the brake releasedevice further comprises a prevention device that prevents the pressureoil accumulated in the brake release accumulator flowing out to thehydraulic power side.
 6. A parking brake unit for a work vehicleaccording to claim 4, further comprising: a brake valve that generatesbrake operating pressure in correspondence with operation of a brakepedal; a foot brake unit that operates in response to the brakeoperating pressure; and a foot brake accumulator that accumulatesspecified pressure oil in order to generate the brake operating pressurein accordance with operation of the brake pedal when an engine isstopped, wherein: the foot brake accumulator also functions as the brakerelease accumulator.
 7. A parking brake unit for a work vehicle,comprising: a hydraulic source; a negative parking brake provided alower portion of the vehicle, that releases brake operation in responseto brake release pressure; an operating member that outputs a releasecommand for the parking brake in response to an operation performed inan operator's cabin; a brake release device that releases operation ofthe parking brake by leading the brake release pressure to the parkingbrake if the release command is output; and a transmission controldevice, wherein: the parking brake is constituted of a transmissionhaving a negative first clutch device and a negative second clutchdevice, the transmission sets a first gear ratio by allowing a propellershaft to rotate if the first clutch device is released by controlpressure for clutch release, and sets a second gear ratio smaller thanthe first gear ratio by allowing the propeller shaft to rotate if thesecond clutch device is released by the control pressure for clutchrelease, the brake release device comprises a brake release accumulatorthat accumulates pressure oil from the hydraulic source, and a controlvalve that allows the pressure oil accumulated in the brake releaseaccumulator to act on one of the first clutch device and the secondclutch device as the brake release pressure if the release command isoutput, the transmission control device comprises a gear ratio controlvalve that controls flow of the control pressure for clutch release foracting on the first clutch device and the second clutch device, and asudden operation prevention accumulator that prevents sudden operationof the parking brake, and the brake release device is provided betweenthe parking brake and the transmission control device.
 8. A parkingbrake unit for a work vehicle according to claim 7, wherein: the brakerelease device further comprises a prevention device that prevents thepressure oil accumulated in the brake release accumulator flowing out tothe hydraulic power side.
 9. A parking brake unit for a work vehicleaccording to claim 7, further comprising: a brake valve that generatesbrake operating pressure in correspondence with operation of a brakepedal; a foot brake unit that operates in response to the brakeoperating pressure; and a foot brake accumulator that accumulatesspecified pressure oil in order to generate the brake operating pressurein accordance with operation of the brake pedal when an engine isstopped, wherein: the foot brake accumulator also functions as the brakerelease accumulator.